Data format for high bit rate wdm transmission

ABSTRACT

The invention includes a method for optically encoding data for transmission over a wavelength division multiplexed optical communications system comprising the steps of: generating a periodic series of optical pulses defining a series of time slots, wherein one pulse appears in each time slot; filtering the pulses to produce carrier pulses extending over more than one time slot; and modulating the pulses with data for transmission. Preferably, the filter gives rise to the pulses having a temporal profile with a minimum in the centre of each of the time slots adjacent to the time slot for that pulse. The resulting data format allows for greater spectral efficiency in a WDM optical transmission system as compared with conventional RZ and NRZ data formats.

FIELD OF THE INVENTION

The present invention relates to optical data transmission in awavelength division multiplex (WDM) scheme, and in particular to aspectrally efficient data format.

BACKGROUND TO THE INVENTION

Transmission of optical data at high bit rates, e.g. 40 Gb/s, in generalbenefits from the use of narrow pulse width optical data. Narrowertemporal pulse width means a broader frequency spectrum for each datapulse, which limits the number of channels that can be used in a WDMscheme. For example, a Return to Zero (RZ) pulse at 10 Gb/s will have aspectral width of about 40 Ghz whilst an RZ pulse at 40 Gb/s will have aspectral width of about 160 GHz.

WDM schemes increase fibre capacity by transmitting multiple channels,each at different wavelengths, over a single fibre. However, opticallyamplified systems have a limited useable bandwidth. Using a higher bitrate increases the bandwidth of each channel and therefore reduces thenumber of channels which can be used. The result is that a bit rate of40 Gb/s offers no advantage over a lower bit rate of say 10 Gb/s becausethe maximum amount of information that can be transmitted in a giventime over an optical fibre link at each rate is approximately the same.

There are a number of schemes which have been developed to improvespectral efficiency at high bit rates, such as using vestigial sideband(VSB) filtering and polarisation division multiplexing, all involvingfurther processing of the optical data signals. The aim of the presentinvention is to provide a data format, and a transmitter and method forproducing the same, which provides improved spectral efficiency overtraditional data formats.

SUMMARY OF THE INVENTION

According to the present invention, a method of optically encoding datafor transmission over a wavelength division multiplexed opticalcommunications system comprises the steps of:

generating a periodic series of optical pulses defining a series of timeslots, wherein one pulse appears in each time slot;

filtering the pulses to produce carrier pulses extending over more thanone time slot; and

modulating the pulses with data for transmission.

The pulses preferably extend over more than one time slot in such a waythat the pulses are resonantly spaced with respect to neighbouringpulses. In other words, the filter bandwidth is selected so that theoscillating tails of the pulses have minima in adjacent time slots.Preferably, the filter gives rise to the pulses having a temporalprofile with a minimum substantially in the centre of each of the timeslots adjacent to the time slot for that pulse. The decision point foreach bit is typically in the centre of the respective time slot and sothe effect on the neighbouring bit should be minimised at that point.Preferably, the filtered carrier pulses have a substantially flat topspectral profile. Preferably, the filter is detuned to optimisetransmission performance.

The step of modulating the pulses with data can be performed eitherbefore or after the filtering step, but is preferably performed beforethe filtering step.

The data resulting from the method of the present invention has arelatively small bandwidth. The effect of overlap between neighbouringbits is mitigated by resonantly positioning the minima of each pulse inthe centre of adjacent pulses.

According to a second aspect of the present invention, a transmitter forproducing an optical data signal for transmission over a wavelengthdivision multiplexed optical communication system comprises:

means for producing a periodic series of optical pulses defining aseries of time slots, wherein one pulse appears in each time slot;

a filter having a spectral profile giving rise to pulses with a temporalprofile extending over more than one time slot; and

modulating means for modulating the pulses with data for transmission.

Preferably, the filter has a substantially flat top spectral profile.Preferably, the filter is detuned to optimise transmission performance.

Preferably, the transmitter includes control means for opticallydetuning the optical filter in order to optimise transmissionperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present invention will now be described in detail withreference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of an optical transmitterarchitecture in accordance with the prior art;

FIG. 2 illustrates an example of a transmitter architecture inaccordance with the present invention;

FIG. 3 illustrates the spectrum of a signal generated in accordance withthe present invention;

FIG. 4 illustrates the temporal profile of a signal generated inaccordance with the present invention; and,

FIG. 5 illustrates an alternative example of a transmitter in accordancewith the present invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of a basic architecture for anoptical transmitter in accordance with the prior art. A coherent lightsource 10, such as a CW laser, produces an optical beam which is firstmodulated with an electrical clock signal using a first modulator 11 andis subsequently modulated with a data signal using a second modulator12. The first and second modulators could be Mach Zehnder (MZ)modulators or another type of electro-optic modulator.

The first modulator 11 provides a series of pulses at a particular bitrate in accordance with the clock signal. The second modulator 12 putsdata onto the series of pulses by modulating the it with NRZ electricaldata. The resulting output is data encoded as an RZ optical signal. AnRZ data format is generally preferred to NRZ data format for long hauloptical transmission as it gives rise to better transmissionperformance.

In WDM transmission schemes, each channel must be spaced from adjacentchannels in the frequency domain in order to avoid inter-channelcrosstalk and other corrupting mechanisms. Each RZ pulse in the datastream of a channel has an associated spectral width. The shorter thepulse i.e. the higher the bit rate, the broader the frequency of thepulse. Conversely, the narrower the spectrum of each pulse the broaderthe pulse in the time domain, which could potentially lead tooverlapping between neighbouring pulses resulting in patterning effects.There is always a balance to be struck between bandwidth and bit rate.

FIG. 2 illustrates a transmitter in accordance with the presentinvention which provides a means for generating optical signals with anarrow spectral width at a particular bit rate whilst avoiding thestrong patterning effects that would be experienced using conventionalRZ data of the same spectral width and bit rate.

In the transmitter in FIG. 2, a pulsed laser light source is used, forexample an active mode locked laser 20. An active mode locked laser canbe made to produce a series of narrow pulses at a particular bit rate.40 Ghz active mode locked lasers are available from a number ofmanufacturers, for example Pritel Inc of Naperville, Ill. USA produce aUOC Series of Ultrafast Optical Clocks suitable for use in the presentinvention.

The pulsed light from the light source is modulated with data using anelectro-optic modulator 21. Electrical NRZ data is written onto thepulsed light stream using a Mach Zehnder modulator driven by anelectrical NRZ data source 22 operating at the same bit rate as thelight source 20.

The pulses from the light source are extremely narrow relative to thebit rate and have a broad spectral profile. However, the transmittershown in FIG. 2 includes a filter element 23 which alters the spectralprofile of the pulses. In order to allow as many channels to be packedinto the available bandwidth, the spectral width of the pulses must bereduced and preferably has a sharp cut-off, i.e. a substantially flattop spectral profile with sharp decay outside the desired frequencyband.

FIG. 3 illustrates the spectrum of a pulse before and after filtering.FIG. 3 a shows the spectrum of a pulse prior to filtering. The pulsesillustrated are 1.7 ps in length and have a frequency content extendingover several hundred Ghz. FIG. 3 b shows the profile of the opticalfilter shown in FIG. 2. The filter is a super-Gaussian 6^(th) orderbandpass filter with a bandwidth of 40 Ghz. Also shown is the idealfilter profile which is a rectangular profile.

FIG. 3 c shows the carrier spectrum of the pulse after filtering withthe filter shown in FIG. 2. The pulse has a spectral profile extendingover only 40 GHz with a sharp decay at each end. FIG. 3 d shows how aseries of channels with this spectral profile can be used in a WDMscheme. Each channel uses filtered pulses with a spectral width of 40GHz and each channel is spaced from adjacent channels by 50 GHz, givinga 10 GHz spacing between the edges of adjacent channels.

FIG. 4 shows the temporal profile of a pulse before and after filtering.FIG. 4 a shows the pulse 40 prior to filtering. FIG. 4 b shows thetemporal profile of the pulse 41 after filtering. The vertical lines 42in FIG. 4 b also show when each time slot begins and ends relative tothe pulse shown. The pulses are produced at 40 GHz in this example andso each time slot lasts 25 ps. It can be clearly seen that the filteredpulse extends over several time slots and that the minima 43 of thefiltered pulse fall in the centre of the time slots adjacent to the timeslot the pulse is centred on. This ensures that the effect of theoverlap on neighbouring bits or pulses is minimised.

The ideal filter profile shown in FIG. 3 b gives rise to a sinc shapedtemporal profile for the carrier pulses having minima in adjacent timeslots. If the bandwidth of the filtered pulses is B then the temporalprofile is of the form sinc (TrBt). However, it is not necessary toproduce pulses with a completely flat top spectral profile to get thebenefit of the present invention, only something approximating to it,such as a super-Gaussian filter described above, which gives rise to acarrier extending over more than one time slot but which has localminima which can be positioned in the centre of time slots adjacent tothe time slot the pulse is centred on.

The transmitter shown in FIG. 2 can be used to produce optical data fortransmission over a single channel in a WDM system. A plurality oftransmitters might be used in a WDM transmitter, one for each channel,with the data streams subsequently multiplexed. The data pulses can bedirectly detected at the receiver end 25 using standard filters.

The error-free transmission distance of the data pulses of the presentinvention is a function of the filter detuning i.e. the asymmetricfilter offset. Ideally, the initial pulses are not filteredsymmetrically about their central frequency. The optimal detuning issensitive to the optical filter shape. For instance, usingsuper-Gaussian filter of the sixth order it can be found that theoptimal detuning is shifted to −6 GHz. Accordingly, the system shown inFIG. 2 includes a control loop 24 and a variable pass band filter. Thebit error rate (BER) is monitored at the receiver end 25 by the controland the filter detuning optimised to minimised the BER.

FIG. 5 shows an alternative transmitter design in accordance with thepresent invention. A coherent light source 50 provides an optical beam.The beam is modulated using an MZ modulator 51 driven with RZ electricaldata 51 at the required bit rate. The data pulses are then amplified byamplifier 53 and passed through a length of nonlinear highly dispersivefibre 54 in order to compress the pulses. The compressed, i.e. narrowedpulses are then filtered using a super-Gaussian type filter 55 as in thetransmitter of FIG. 2. The control loop for the filter detuning is notshown but is equally applicable to this transmitter as it is to thetransmitter of FIG. 2.

It should be noted that the system shown in FIG. 1 might be suitable forproducing narrow pulses which could be subsequently filtered inaccordance with the present invention. In order to produce the requirednarrow pulses the modulator 11 would have to be able to switch on andoff very quickly. Suitable modulators may be available in the nearfuture.

The present invention provides a data format that is tolerant to overlapbetween neighbouring bits, allowing greater spectral efficiency in a WDMtransmission scheme. The fact that each data pulse extends across morethan one time slot does not destroy the data. The pulse shape can bechosen for a particular application such that the effect of the overlapis tolerable, whilst maximising spectral efficiency.

1. A method of optically encoding data for transmission over awavelength division multiplexed optical communications system comprisingthe steps of: generating a periodic series of optical pulses defining aseries of time slots, wherein one pulse appears in each time slot;filtering the pulses to produce carrier pulses extending over more thanone time slot; and modulating the pulses with data for transmission;wherein the filter gives rise to the pulses having a temporal profilewith a minimum substantially in the center of each of the time slotsadjacent to the time slot for that pulse.
 2. (canceled)
 3. A methodaccording to claim 1, wherein the filtered carrier pulses have asubstantially flat top spectral profile.
 4. A method according to claim1 or 2, wherein the filter is detuned to optimise transmissionperformance.
 5. A method according to any preceding claim, wherein thestep of modulating the pulses with data is performed before thefiltering step.
 6. A transmitter for producing an optical data signalfor transmission over a wavelength division multiplexer opticalcommunication system comprising: means for producing a periodic seriesof optical pulses defining a series of time slots, wherein one pulseappears in each time slot; a filter having a spectral profile givingrise to pulses with a temporal profile extending over more than one timeslot, the temporal profile having a minimum substantially in the centerof each of the time slots adjacent to the time slot for that pulse; andmodulating means for modulating the pulses with data for transmission.7. A transmitter according to either claim 5 or 6, wherein the filterhas a substantially flat top spectral profile.
 8. A transmitteraccording to either claim 5 or 6, wherein the filter is detuned tooptimise transmission performance.
 9. A transmitter according to claim7, further comprising control means for optically detuning the opticalfilter in order to optimise transmission performance.